The present invention relates to novel compounds that are analogs of glutamic acid and gamma-aminobutyric acid (GABA). More specifically, the analogs are useful as antiseizure therapy for central nervous system disorders such as epilepsy, Huntington""s chorea, cerebral ischemia, Parkinson""s disease, tardive dyskinesia, and spasticity. It is also possible that the present invention could be used as an antidepressant, anxiolytic, and antipsychotic activity.
Gamma aminobutyric acid (GABA) and glutamic acid are two major neurotransmitters involved in the regulation of brain neuronal activity. GABA is the major inhibitory neurotransmitter and L-glutamic acid is an excitatory transmitter (Roberts E, et al, GABA in Nervous System Function, Raven Press: New York, 1976; McGeer E G, et al, Glutamine, Glutamate, and GABA in the Central Nervous System; Hertz L, Kvamme E, McGeer E G, Schousbal A, eds., Liss: New York, 1983;3-17). An imbalance in the concentration of these neurotransmitters can lead to convulsive states. Accordingly, it is clinically relevant to be able to control convulsive states by controlling the metabolism of this neurotransmitter. When the concentration of GABA diminishes below a threshold level in the brain, convulsions result (Karlsson A, et al, Biochem. Pharmacol 1974;23:3053-3061). When the GABA levels rise in the brain during convulsions, seizures terminate (Hayashi T J, Physiol. (London) 1959;145:570-578). The term seizure as used herein means excessive unsynchronized neuronal activity that disrupts normal neuronal function. In several seizure disorders there is concomitant with reduced brain GABA levels a diminished level of L-glutamic acid decarboxylase (GAD) activity also observed (McGeer P O, et al, In: GABA in Nervous System Function; Roberts E, Chase T N, Tower D B, eds., Raven Press: New York 1976:487-495; Butterworth J, et al, Neurochem. 1983;41:440-447; Spokes E G, Adv. Exp. Med. Biol. 1978;123:461-473; Wu J Y, et al, Neurochem. Res. 1979;4:575-586; and Iversen L L, et al, Psychiat. Res. 1974;11:255-256). Often, the concentrations of GAD and GABA vary in parallel because decreased GAD concentration results in lower GABA production.
Because of the importance of GABA as an inhibitory neurotransmitter, and its effect on convulsive states and other motor dysfunctions, a variety of approaches have been taken to increase the brain GABA concentration. For example, the most obvious approach was to administer GABA. When GABA is injected into the brain of a convulsing animal, the convulsions cease (Purpura D P, et al, Neurochem. 1959;3:238-268). However, if GABA is administered systematically, there is no anticonvulsant effect because GABA, under normal circumstances, cannot cross the blood brain barrier (Meldrum B S, et al, Epilepsy; Harris P, Mawdsley C, eds., Churchill Livingston: Edinburg 1974:55. In view of this limitation, there are three alternative approaches that can be taken to raise GABA levels.
The most frequent approach is to design a compound that crosses the blood brain barrier and then inactivates GABA aminotransferase. The effect is to block the degradation of GABA and thereby increase its concentration. Numerous mechanism-based inactivators of GABA aminotransferase are known (Silverman R B, Mechanism-Based Enzyme Inactivation: Chemistry and Enzymology, Vol. I and Il, CRC: Boca Raton 1988).
Another approach is to increase GABA concentrations in the brain by making GABA lipophilic by conversion to hydrophobic GABA amides (Kaplan J P, et al, G.J. Med. Chem. 1980;23:702-704; Carvajal G, et al, Biochem, Pharmacol. 1964;13:1059-1069; Imines: Kaplan J P, Ibid.; or GABA esters: Shashoua V E, et al, J. Med. Chem. 1984;27:659-664; and PCT Patent Application WO85/00520, published Feb. 14, 1985) so that GABA can cross the blood brain barrier. Once inside the brain, these compounds require amidase and esterases to hydrolyze off the carrier group and release GABA.
Yet another approach is to increase 25 brain GABA levels by designing an activator of GAD. A few compounds have been described as activators of GAD. The anticonvulsant agent, maleicimid, was reported to increase the activity of GAD by 11% and as a result increase GABA concentration in the substantia nigra by up to 38% (Janssens de Varebeke P, et al, Biochem. Pharmacol. 1983;32:2751-2755. The anticonvulsant drug sodium valproate (Loscher W, Biochem. Pharmacol. 1982;31:837-842; Phillips N I, et al, Biochem. Pharmacol. 1982;31:2257-2261) was also reported to activate GAD and increase GABA levels.
The compounds of the present invention have been found to activate GAD in vitro and have a dose dependent protective effect on-seizure in vivo.
Also, the compounds of the present invention have been found to bind a novel binding site which was identified to bind tritiated gabapentin. Gabapentin has been found to be an effective treatment for the prevention of partial seizures in patients refractory to other anticonvulsant agents. Chadwick D, Gabapentin, pp. 211-222, In: Recent Advances in Epilepsy, Vol. 5, Pedley T A, Meldrum B S, (eds.) Churchill Livingstone, New York (1991). The novel binding site labeled by tritiated gabapentin was described in membrane fractions from rat brain tissue and in autoradiographic studies in rat brain sections, Hill D, Ibid. This binding site has been used to evaluate the compounds of the present invention.
The novel compounds of the present invention are set forth below as Formula I. It should be noted that the compound of Formula I wherein R1 is methyl and each of R2 and R3 is hydrogen is taught in Japan Patent Number 49-40460.
In accordance with the present invention, there is provided compounds of the Formula I 
wherein R1 is a straight or branched alkyl of from 1 to 6 carbons, phenyl or cycloalkyl having from 3 to 6 carbon atoms; R2 is hydrogen or methyl; and R3 is hydrogen, methyl, or carboxyl; with the proviso that when each of the R2 and R3 is hydrogen, R1 is other than methyl. Pharmaceutically acceptable salts of the compounds of Formula I are also included within the scope of the present invention. Also included within the scope of the present invention are the individual enantiomeric isomers of the compounds of the Formula I.
The present invention also provides pharmaceutical compositions of the compounds of Formula I.
Also provided as a part of the present invention is a novel method of treating seizure disorders in a patient by administering to said patient an anticonvulsant effective amount of a compound of the following Formula II 
wherein R11 is a straight or branched alkyl of from 1 to 6 carbon atoms, phenyl, or cycloalkyl having from 3 to 6 carbon atoms; R12 is hydrogen or methyl; and R13 is hydrogen, methyl, or carboxyl; or an individual enantiomeric isomer thereof; or a pharmaceutically acceptable salt thereof.
Also, the present invention provides a method for increasing brain neuronal GABA and provides pharmaceutical compositions of the compounds of Formula II.
The present invention provides novel processes for the synthesis of chiral Formula I compounds.
In accordance with the present invention, there is provided a series of 3-alkyl-4-aminobutyric acid or 3-alkyl glutamic acid analogs which are useful as anticonvulsants. Illustrative of the alkyl moieties as represented by R1 and R11 in Formulas I and II are methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, isopentyl, and neopentyl as well as other alkyl groups. The cycloalkyl groups represented by R1 and R11 in Formulas I and II are exemplified by cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl. The analogs are further shown herein to prevent seizure while not causing the side effect of ataxia, such a side effect being found in several anti-seizure pharmaceuticals.
The more preferred compounds of the present invention are of Formula I above wherein R3 is hydrogen, R2 is hydrogen, and R1 isobutyl.
That is, the preferred compound is 4-amino-3-(2-methylpropyl)butanoic acid. It has been found that this compound is unexpectedly more potent than the other analogs synthesized in accordance herewith and tested in vivo. What is further surprising, as the following data shows, is that this preferred compound is the least effective one of the analogs tested in activating GAD in vitro. Accordingly, it was very unexpected that this preferred compound had such a high potency when tested in vivo.
The most preferred compounds of the present invention are the (S)-(+)- and the (R)(xe2x88x92)-4-amino-3-(2-methylpropyl)butanoic acid with the (S)-(+)-enantiomer being most preferred. The (S)-(+)-enantiomer was found to be the most potent compound within the scope of the present invention for displacement of tritiated gabapentin, and both the (S)-(+)- and the (R)-(xe2x88x92)-enantiomers showed pronounced stereoselectivity for both displacement of tritiated gabapentin and for anticonvulsant activity in vivo.
The compounds made in accordance with the present invention may form pharmaceutically acceptable salts with both organic and inorganic acids or bases. For example, the acid addition salts of the basic compounds are prepared either by dissolving the free base in aqueous or aqueous alcohol solution or other suitable solvents containing the appropriate acid and isolating the salt by evaporating the solution. Examples of pharmaceutically acceptable salts are hydrochlorides, hydrobromide, hydrosulfates, etc, as well as sodium, potassium, and magnesium, etc, salts.
The method for the formation of the 3-alkyl-4-aminobutanoic acids starting from 2-alkanoic esters is prepared from commercially available aldehydes and monomethyl malonate by the Knoevenagel reaction, (Kim Y C, et al, J. Med. Chem. 1965:8509) with the exception of ethyl 4,4-dimethyl-2-pentenoate.
More specifically, the following is a procedure which can be generally applied to the preparation of all the 3-alkylglutamic acids. Ten grams of a 3-alkyl-5,5-dicarbethoxy-2-pyrrolidinone was refluxed in 150 mL of 49% fuming HBr for 4 hours. After this time, the contents were placed in an evaporator and the volatile constituents were removed in vacuo with the aid of a hot-water bath. The gummy residue was dissolved in 25 mL of distilled water and the water was removed with the aid of the evaporator. This process was repeated once more. The residue was dissolved in 20 mL of water, and the pH of the solution was adjusted to 3.2 with concentrated NH3 solution. At this point the chain length of the individual 3-alkylglutamic acids altered the solubility so that those whose side chains were larger precipitated with the ease from solution. Precipitation of the alkylglutamic acids with smaller substituents (methyl, ethyl, and propyl) could be encouraged by cooling on an ice bath or by diluting the aqueous solution with 100 mL of absolute ethanol. Precipitation from the water-alcohol mixture is complete in 48 hours. Care must be taken to add the ethanol slowly to prevent the precipitation of an amorphous solid which is not characteristic of the desired 3-alkylglutamic acids. Samples of the amino acids were purified for analysis by recrystallizing from a water-ethanol mixture. All melted with decomposition. Melting points of the decomposed 3-alkylglutamic acids corresponded with those of their pyroglutamic acids.
Ethyl 4,4-dimethyl-2-pentenoate was prepared from 2,2-dimethylpropanol and ethyl lithioacetate, followed by dehydration of the xcex2-hydroxy ester with phosphoryl chloride and pyridine.
The Michael addition of nitromethane to alpha, xcex2-unsaturated compounds mediated by 1,1,3,3-tetramethylguanidine or 1,8-diazabicyclo[5.4.0]undec-7-ene(DBU) afforded 4-nitroesters in good yields. More specifically, a mixture of nitromethane (5 mol), xcex1,xcex2-unsaturated ester (1 mol), and tetramethyl-guanidine (0.2 mol) was stirred at room temperature for 2 to 4 days. (In case of methyl acrylate, the ester has to be added at a temperature below 300.) The progress of the reaction was followed by IR (disappearance of the Cxe2x95x90C band) and G.L.C. analysis. The reaction mixture was washed with dilute hydrochloric acid and extracted with ether. The organic extract was dried, the solvent removed at reduced pressure, and the 20 residue distilled at a pressure of 2 torr. Although the aliphatic nitro compounds are usually reduced by either high pressure catalytic hydrogenation by metal-catalyzed transfer hydrogenation, or by newly introduced hydrogenolysis methods with ammonium formate or sodium borohydride and palladium as catalysts, applicants have found that 4-nitrocarboxylic esters can be reduced almost quantitatively to the corresponding 4-aminocarboxylic esters by hydrogenation using 10% palladium on carbon as catalysts in acetic acid at room temperature and atmospheric pressure. The amino esters produced were subjected to acid hydrolysis to afford the subject inventive compounds in good yields. This procedure provides access to a variety of 3-alkyl-4-aminobutanoic acids as listed in Tables 1 and 2 as examples and thus is advantageous in comparison to methods previously used.
Examples of more specific methods of making compounds in accordance with the present invention are as follows, optionally utilizing the methods described in detail above. When the starting material is not commercially available, the synthetic sequence may be initiated with the corresponding alcohol, which is oxidized to the aldehyde by the method of Corey E J, et al, Tetrahedron Lett. 1975:2647-2650.
The chiral compounds of Formulas I and II are prepared as set forth in the schematic in Chart I hereof. Although the schematic in Chart I depicts the chiral synthesis of specific compound (S)-(+)-4-amino-3-(2-methylpropyl)butanoic acid, one skilled in the art can readily see that the method of synthesis can be applied to any diastereomeric compound of Formulas I and II.
In Chart I Ph is phenyl, Bn is benzyl, THF is tetrahydrofuran, LDA is lithium diisopropylamide, BH3.SMe2 is borane dimethyl sulfide complex, TsCl is tosyl chloride, and DMSO is dimethylsulfoxide.
The detailed synthetic procedure is set forth hereinbelow in Example 1. The key introductory literature for this methodology was discussed in Evans"" paper, J. Am. Chem. Soc. 1982;104:1737-9. The metal enolate can be formed with a lithium or sodium amide base, and subsequently alkylated to give an a substituted carboxylic acid derivative. This methodology was valuable for the enantioselective synthesis of these xcex1-substituted carboxylic acid derivatives. In this seminal paper, Evans described the preparation of propionic acid derivatives with a series of simple alkylating agents. By varying the stereochemistry of the chiral synthon (the oxazolidinone), he was able to get high stereoselectivity.
Evans has used this chiral auxiliary in other synthetic studies, but none has been related to 4-amino-3-(2-methylpropyl)butanoic acid which contains a xcex2-substituted-xcex3-amino acid. The methodology as presented by Evans teaches toward xcex1-substitution, and away from xcex2-substitution, and has not been used in the preparation of this type of unusual amino acid. N-acyloxazolidinones have been used to form chlorotitanium enolates that have been reacted with Michael adducts such as acrylonitrile, J. Org. Chem. 1991;56:5750-2. They have been used in the synthesis of the rutamycin family of antibiotics, J. Org. Chem. 1990;55:6260-8 and in stereoselective aldol condensations, Org. Synth. 1990;68:83-91. Chiral xcex1-amino acids were prepared via the oxazolidinone approach. In this sequence, a dibutylboron enolate was brominated and displaced with azide, Tetrahedron Lett. 1987;28:1123-6. Other syntheses of xcex2-hydroxy-xcex1-amino acids were also reported via this chiral auxiliary through aldol condensation (Tetrahedron Lett. 1987;28:39-42; J. Am. Chem. Soc. 1987;109:7151-7). xcex1,xcex2-Unsaturated N-acyloxazolidinones have also been used to induce chirality in the Diels-Alder reaction, (J. Am. Chem, Soc. 1988;110:1238-56. In none of these examples, or others found in the literature, is this methodology used to prepare (xcex2-substituted carboxylic acids or 3-substituted GABA analogs.
In another embodiment, the chiral compounds of Formulas I and II can be prepared in a manner which is similar to the synthesis depicted in Chart I. In this embodiment, however, step 8 in Chart I is replaced by an alternate two step procedure which is set forth hereinbelow in Example 2 (sodium hydroxide is preferred, however, other solvents known to those of skill in the art which can hydrolyze the azide (8) to intermediate azide (8a) can be employed). Instead of reducing the azide (8) to the amino acid (9) in Chart I, the alternate procedure hydrolyzes the azide (8) to give an intermediate azide (8a) which is subsequently reduced (see Chart Ia).
There are two major advantages to hydrolyzing azide (8) to give the intermediate azide (8a) prior to reduction. The first advantage is that intermediate azide (8a) may be purified by extraction into aqueous base. After the aqueous extract is acidified, intermediate azide (8a) may be extracted into the organic phase and isolated. This allows for a purification of intermediate azide (8a) which does not involve chromatography. The purification of azide (8) requires chromatography which is very expensive and often impractical on a large scale.
The second advantage is that intermediate azide (8a) may be reduced to amino acid (9) without added acid. Reduction of azide (8) requires addition of acid, e.g., hydrochloric acid in order to obtain amino acid (9). Unfortunately, lactamization of amino acid (9) is promoted by the presence of acid. Intermediate azide (8a) may be reduced under near neutral conditions to give amino acid (9), thus minimizing the problem of lactam formation.
In another preferred embodiment, the chiral compounds of Formulas I and II can be prepared as set forth in the Schematic in Chart II hereof. Although the schematic in Chart II depicts the chiral synthesis of specific compound (S)-(+)-4-amino-3-(2-methyl-propyl)butanoic acid, one skilled in the art can readily see that the method of synthesis can be applied to any diastereomeric compound of Formulas I and II.
In Chart II Ph is phenyl, and Ts is tosyl.
The detailed synthetic procedure is set forth hereinbelow in Example 3. This procedure is similar to the synthesis route depicted in Chart I, however, the procedure of Chart II replaces the benzyl ester in the synthesis route of Chart I with a t-butyl ester. The desired amino acid (9) and (109) is the same end product in both Charts I and II, respectively.
There are several advantages to using the t-butyl ester rather than the benzyl ester in the synthesis of amino acid (9) or (109). A first advantage relates to the hydrolysis of the chiral auxiliary in step 4 of Chart 1. During the hydrolysis of the chiral auxiliary in this reaction some hydrolysis of the benzyl ester often occurs. Hydrolysis of the t-butyl ester in Chart II has not been experienced.
Another advantage relates to the use of alcohol (106) in Chart II over the use of alcohol (6) in Chart I. A problem with the benzyl ester-alcohol is the tendency of the benzyl ester-alcohol to undergo lactonization as shown below. Although lactonization of the benzyl ester can be avoided under some conditions, the t-butyl ester-alcohol is far less prone to lactonization. 
Still another advantage, which was previously discussed with regard to the synthetic procedure depicted by Chart Ia, is that the t-butyl synthetic route minimizes the problem of lactam formation of the amino acid end product (109). Instead of reducing azide (108) to amino acid (109) which requires the addition of acid that causes lactamization of amino acid (109), azide (108) is first hydrolyzed to intermediate azide (108a). Intermediate azide (108a) may be reduced under neutral conditions to give amino acid (109), thus minimizing the problem of lactam formation.
It should also be mentioned that several novel intermediates are produced by the processes discussed herein. Some of these intermediates which are depicted in Charts I, Ia, and II include in the racemate or R or S enantiomer form:
4-methyl-5-phenyl-2-oxazolidinone,
4-methyl-(2-methylpropyl)-2-dioxo-5-phenyl-3-oxazolidine butanoic acid, phenylmethyl ester,
4-methyl-pentanoyl chloride,
4-methyl-3-(4-methyl-1-oxopentyl)-5-phenyl-2-oxazolidinone,
2-(2-methylpropyl)-butanedioic acid, 4-(phenylmethyl)ester,
3-(azidomethyl)-5-methyl-hexanoic acid, phenylmethyl ester,
3-(hydroxymethyl)-5-methyl-hexanoic acid, phenylmethyl ester,
5-methyl-3-[[[(4-methylphenyl)sulfonyl]oxy]methyl]-hexanoic acid, phenylmethyl ester,
3-(azidomethyl)-5-methyl-hexanoic acid,
2-(2-methylpropyl)-1,4-butanedioic acid, 4-(1,1-dimethylethyl)ester,
3-(azidomethyl)-5-methyl-, 1,1-dimethylethyl ester,
3-(hydroxymethyl)-5-methyl-hexanoic acid, 1,1-dimethyl ester,
5-methyl-3-[[[(4-methyl(phenyl)sulfonyl]oxy]methyl-hexanoic acid, 1,1-dimethylethyl ester, or
4-methyl-(2-methylpropyl)-2-dioxo-5-phenyl-3-oxazolidinebutanoic acid, 1,1-dimethylethyl ester.
The compounds made by the aforementioned synthetic methods can be used as pharmaceutical compositions as an antidepressant, anxiolytic, antipsychotic, antiseizure, antidyskinesic, or antisymptomatic for Huntington""s or Parkinson""s diseases when an effective amount of a compound of the aforementioned formula together with a pharmaceutically acceptable carrier is used. That is, the present invention provides a pharmaceutical composition for the suppression of seizures resulting from epilepsy, the treatment of cerebral ischemia, Parkinson""s disease, Huntington""s disease and spasticity and also possibly for antidepressant, anxiolytic, and antipsychotic effects. These latter uses are expected due to functional similarities to other known compounds having these pharmacological activities. The pharmaceutical can be used in a method for treating such disorders in mammals, including human, suffering therefrom by administering to such mammals an effective amount of the compound as described in Formulas I and II above in unit dosage form.
The pharmaceutical compound made in accordance with the present invention can be prepared and administered in a wide variety of dosage forms. For example, these pharmaceutical compositions can be made in inert, pharmaceutically acceptable carriers which are either solid or liquid. Solid form preparation include powders, tablets, dispersible granules, capsules, cachets, and suppositories. Other solid and liquid form preparations could be made in accordance with known methods of the art. The quantity of active compound in a unit dose of preparation may be varied or adjusted from 1 mg to about 300 mg/kg (milligram per kilogram) daily, based on an average 70 kg patient. A daily dose range of about 1 mg to about 50 mg/kg is preferred. The dosages, however, may be varied depending upon the requirement with a patient, the severity of the condition being treated, and the compound being employed. Determination of the proper dosage for particular situations is within the skill of the art.
Illustrative examples of compounds made in accordance with the present invention were tested to demonstrate the ability of the compounds to activate GAD in vitro and to prevent seizure in vivo without the side effect of ataxia.
In Vitro GAD Activation
Assays were carried out in 10 mL vials sealed with serum caps through which a center well (Kontes Catalog No. 882320-000) was inserted. The center well was charged with 200 xcexcL of freshly prepared 8% KOH solution. Various concentrations of L-glutamic acid (0.5, 0.25, 0.166, 0.125, 0.10 mM) containing [14C]L-glutamate (10 xcexcCi/mmol) in 50 mM potassium phosphate buffer, pH 7.2 were shaken at 37xc2x0 C. in separate vials with purified L-glutamic acid decarboxylase (18.75 xcexcg; spec. act 10.85 xcexcmol/min mg) in a total volume of 2.00 mL. After being shaken for 60 minutes, the enzyme reactions were quenched by the addition of 200 xcexcL of 6 M sulfuric acid to the contents of each of the vials. The vials were shaken for an additional 60 minutes at 37xc2x0 C. The center wells were removed and placed in scintillation vials with 10 mL of scintillation fluid for radioactivity determination. The same assays were repeated except in the presence of various concentrations of the activators (2.5, 1.0, 0.5, 0.25, 0.1, 0.05 mM). The Vmax values were determined from plots of 1/cpm versus 1/[glutamate] at various concentrations of activators. The data were expressed as the ratio of the Vmax in the presence of the activators to the Vmax in the absence of the activators times 100%.
The results of the experiment are shown in Table 1. The tests show that there was significant activation by the various compounds tested to differing degrees. The known activator sodium valproate and gabapentin were tested.
In vivo tests were performed to demonstrate the seizure preventing capabilities of the novel compounds. Threshold maximum electroshock is an animal model test for generalized seizures that is similar to that of Piredda S G, et al, Pharmacol. and Exptl. Therap. 1985;232(3):741-45. The methods for this test are described as follows.
Male CF-1 mice (22-30 g) were allowed free access to food and water prior to testing. For screening, groups of five mice were given a compound intravenously at doses of 30, 100, and 300 mg/kg and tested at 0.5, 2.0, and 4.0 hours after dosing. Drugs were either dissolved in 0.9% saline or suspended in 0.2% methylcellulose. Animals were shocked with corneal electrodes (see below) and observed for tonic hindlimb extensor seizures. Absence of hindlimb extension was taken as an anticonvulsant effect.
The electroshock apparatus delivered a 60 Hz sine wave with a current amplitude of 14 mA (peak-to-peak) for 0.2 seconds. The current strength of 14 mA used in this procedure produced tonic extensor seizures in approximately 95% of untreated mice, but was only slightly above threshold for tonic extension.
Summaries of the numbers of animals protected from seizures when tested 120 minutes after administration of each compound set forth in the left-hand column are given in Table 2 for varying dose levels set forth in the second column of the table.
Due to the interesting phenomena related to the (R,S)-i-butyl GABA (the compound having significantly higher potency and effectiveness without causing ataxia), threshold maximal electroshock tests where conducted varying the time of testing from 1 hour to 8 hours, the dose being 10 mg/kg in mice, injected intravenously. Table 3 shows the results of these tests indicating a maximum protection after 2 hours of testing.
In view of the above results, a dose response curve was made for the two hour testing time period in mice, the drug being given intravenously at 10 mg/kg. The results of this test is shown in Table 4 with a calculated ED50 equaling 2.07 mg/kg.
A third pharmacological test was performed as described in Krall R L, et al, Epilepsia. 1978;19:409. In this procedure, drugs were tested for attenuation of threshold clonic seizures in mice caused by subcutaneous administration of pentylenetetrazol (85 mg/kg) which is a generally accepted model for absence type seizures. Results from the third test for the compound when administered either intravenously or orally is shown in Table 5. The test was conducted at three dose levels, showing effective protection at 30 mg/kg and 100 mg/kg with no ataxia.
The above is a significant finding because the compound having the least ability to activate GAD in vitro surprisingly had an approximately 10-fold increase in potency over the other compounds tested. Even more unexpected is the absence of ataxic side effect coupled to this increase in potency.
As noted hereinabove, the S-(+)enantiomer of 4-amino-3-(2-methylpropyl)butanoic acid (3-isobutyl GABA or IBG) which is structurally related to the known anticonvulsant, gabapentin, potently displaces tritiated gabapentin from a novel high-affinity site in rat brain membrane fractions. Also the S-(+)enantiomer of 3-isobutyl GABA is responsible for virtually all blockade of maximal electroshock seizures in mice and rats. The R(xe2x88x92)enantiomer of 3-isobutyl GABA is much less effective in the blockade of maximal electroshock seizures and in displacement of tritiated gabapentin from the novel high-affinity binding site. Table 6 below sets forth data comparing gabapentin, racemic 3-isobutyl GABA ((xc2x1)-IBG), S-(+)-3-isobutyl GABA ((S)-IBG) and R-(xe2x88x92)-3-isobutyl GABA ((R)-IBG) in these assays.
The data set forth in Table 6 was obtained as follows. For anticonvulsant testing, male CF-1 strain mice (20-25 g) and male Sprague-Dawley rats (75-115 g) were obtained from Charles River Laboratories and were maintained with free access to food and water before testing. Maximal electroshock was delivered with corneal electrodes by conventional methods (Krall, supra, 1975) except that low-intensity electroshock with mice consisted of 17 mA of current rather than the conventional 50 mA (zero to peak). Briefly, mice were given test substance and were tested for prevention of seizures by application of electrical current to the corneas by 2 metal electrodes covered with gauze and saturated with 0.9% sodium chloride. Electroshock stimulation was delivered by a constant-current device that produced 60 Hz sinusoidal electrical current for 0.2 seconds. For rats, maximal electroshock stimulation consisted of 120 mA of current. Ataxia in mice was assessed by the inverted screen procedure in which mice were individually placed on a 4.0-inch square of wire mesh that was subsequently inverted (Coughenour, supra, 1978). Any mouse that fell from the wire mesh during a 60 second test period was rated as ataxic. ED50 values were determined by probit analysis of results with at least 5 dose groups of 10 mice or 8 rats each.
All drugs were freely soluble in aqueous media. For in vivo studies, drug solutions were made in 0.9% sodium chloride and given in a volume of 1 mL/100 g body weight. Intravenous administration was given by bolus injection into the retro-orbital sinus in mice. Oral administrations were by intragastric gavage.
For binding studies, partially purified synaptic plasma membranes were prepared from rat neocortex using sucrose density gradients. The cerebral cortex of 10 rats was dissected from the rest of the brain and homogenized in 10 volumes (weight/volume) of ice-cold 0.32 M sucrose in 5 mM tris-acetate (pH 7.4) using a glass homogenizer fitted with a teflon pestle (10-15 strokes at 200 rpm). The homogenate was centrifuged at 100 g for 10 minutes and the supernatant collected and kept on ice. The pellet (PI) was rehomogenized in 20 mL of tris-sucrose and the homogenate recentrifuged. The combined supernatants were centrifuged at 21,500 g for 20 minutes. The pellet (P2) was resuspended in 1.2 M tris-sucrose and 15 mL of this mixture was added to ultracentrifuge tubes. On to this, 10 mL of 0.9 M sucrose was layered followed by a final layer of 5 mM tris-acetate, pH 8.0. Tubes were centrifuged at 100,000 g for 90 minutes. The synaptic plasma membranes located at the 0.9/1.2 M sucrose interface were collected, resuspended in 50 mL of 5 mM tris-acetate, pH 7.4, and centrifuged at 48,000 g. The final pellet was resuspended in 50 mL of tris-acetate, pH 7.4, aliquoted, and then frozen until use.
The assay tissue (0.1 to 0.3 mg protein) was incubated with 20 mM [3H]-gabapentin in 10 mM HEPES buffer (pH 7.4 at 20xc2x0 C., sodium free) in the presence of varying concentrations of test compound for 30 minutes at room temperature, before filtering onto GFB filters under vacuum. Filters were washed 3 times with 5 mL of ice cold 100 mM NaCl solution and dpm bound to filters was determined using liquid scintillation counting. Nonspecific binding was defined by that observed in the presence of 100 mM gabapentin.
In view of the above demonstrated activity of the compounds characterizing the present invention and in particular the 4-amino-3-(2-methylpropyl)butanoic acid (isobutyl GABA) the compounds made in accordance with the present invention are of value as pharmacological agents, particularly for the treatment of seizures in mammals, including humans.